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Analysis of CFD Modeling Techniques Over the MV-22 Tiltrotor

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Analysis of CFD Modeling Techniques Over the MV-22 Tiltrotor Analysis of CFD Modeling Techniques over the MV-22 Tiltrotor Jennifer Abras Robert Narducci Aerospace Engineer Boeing Associate Technical Fellow NAVAIR 4.3.2.1 The Boeing Company Patuxent River, MD 20670 Philadelphia, PA 19142 [email protected] [email protected] ABSTRACT The V-22 Osprey is a tiltrotor aircraft designed to operate under a wide range of flight conditions. Its outer mold line geometry is aerodynamically complex in part because aerodynamic considerations were not primary influential factors for the major features of the aircraft. As mission requirements change and additional devices are added to the aircraft, questions regarding the aerodynamic impact must be answered. While many of these questions can be adequately answered using lower-fidelity methods, some situations require the use of higher-fidelity analysis. Computational fluid dynamics (CFD) is a tool that has been used frequently to answer aerodynamic questions associated with the V-22. However, the complexity of the aircraft makes this analysis challenging. Using unstructured grids is one way of reducing the lead time required to setup the simulation as unstructured grids lend themselves to modeling complex geometries. This paper compares independent OVERFLOW and FUN3D CFD analyses of the MV-22 tiltrotor in airplane mode over a range of angles of attack, and compares these results to data from a recent high-angle-of-attack wind tunnel test run in the Boeing V/STOL Wind Tunnel. The results lend insight into the choice of grid structure, near-body vortex generation, and numerical methodology, and reveal that care must be taken when setting up the CFD model as well as identifying any numerical phenomena that could be considered code specific. Much of the early testing focused on the full- INTRODUCTION scale development (FSD) aircraft. The current MV-22 The V-22 Osprey (Figure 1) is a multi-service, Block B configuration has several shape changes multi-mission tiltrotor aircraft capable of operating in including nacelle and spinner; and fuselage sponson. many diverse flight conditions. It is designed to take-off Many avionic kits and antennae are housed in fairings that and land as a helicopter and fly like a turboprop airplane. protrude throughout the fuselage. Recently The Boeing During the design phase, the overall dimensions of the Company re-established its wind tunnel database for the aircraft were defined by shipboard compatibility MV-22 (Marines) and CV-22 (Air Force) variants in an requirements (for example to operate, fold and stow on an entry at the 20 FT x 20 FT Boeing V/STOL Wind Tunnel LHA-class ship; and to carry an F-404 engine (BVWT)4,5. A photograph of the MV-22 model is shown internally1). The wing thickness, sweep, and dihedral in Figure 2. were also determined by non-aerodynamic considerations2. Extensive wind-tunnel testing in the 1980s helped shape the wing-fuselage juncture, overwing fairing, rear fuselage upsweep, and tail configuration. Figure 2. MV-22 BVWT 0468 high-angle-of-attack Figure 1. The V-22 Osprey3. 0.15-scale wind tunnel model. The V-22 Osprey is in full production and has flown many operational missions for the Marines in Iraq Presented at the American Helicopter Society 66th Annual and the Air Force in Afghanistan. As it’s capability is Forum, Phoenix, AZ, May 11-13, 2010. This is a work of the being verified and realized in fleet operations, the U.S. Government and is not subject to copyright protection in Marines and Air Force are actively persuing definition of the U.S. new missions to take advantage of it’s full potential. To APPROACH support new designs, conventional aerodynamic tools and Both unstructured (FUN3D) and structured handbook assessments are often used to estimate drag or (OVERFLOW) CFD approaches are compared and air loads and to size new parts. For aerodynamic benchmarked against new wind tunnel measurements interference effects, higher-fidelity analysis such as CFD obtained for the MV-22. is often utilized. Wind tunnel testing or flight test demonstrations are conducted whenever the level of uncertainty exceeds the ability of analytical means to FUN3D 10.8 predict accurately. However, these tests are expensive FUN3D9,10 is an unstructured CFD code and generally have schedule implications. developed at NASA Langley that solves the RANS equations using a node-centered 2nd-order upwind implicit Computational fluid dynamics (CFD) offers scheme. While there a variety of flux schemes and practical and timely analysis to address many turbulence models that are available, for this study the aerodynamic designs. Advances in computer technology Roe flux scheme and the Spalart-Allmaras turbulence and computational algorithms have allowed Reynolds- model have been selected, since this turbulence model is Averaged Navier-Stokes (RANS) solvers to be used available in both CFD codes studied. Additional relevant routinely in most aircraft programs. The V-22 Program is settings used within FUN3D consist of steady-state no exception and has relied on CFD for the past 15 years pseudo time-stepping formulation as well as mapped to provide results for the analysis of the many flow least-squares11. The mapped LSQ option is applied to regimes of the tiltrotor aircraft. Today, computational reduce errors seen in unweighted LSQ methods applied to models with tens of millions of grid cells are common and high aspect ratio elements near curved boundaries. It was lead to very accurate and sophisticated flow simulations. ultimately found not to have a significant impact on the CFD simulations for airplane-mode operations were results. studied, for example by Tai6, in the late 1990s. More recently, advances in CFD modeling to capture the OVERFLOW 2.1 rotation of rotor blades have allowed researchers to produce very sophisticated hover simulations. Potsdam OVERFLOW12 is also a RANS code developed studied rotor-fuselage interaction of the V-22 in hover at NASA Langley; however solutions are computed on capturing the asymmetric fountain effect7. Using a less structured overset grids. In this application of sophisticated rotor model Narducci et al. modeled the OVERFLOW, a 2nd-order finite-volume central interaction of two V-22 aircraft in shipboard operations8. differencing scheme with scalar dissipation is utilized with the Spalart-Allmaras turbulence model. Steady-state Although every attempt was made to optimize solutions are obtained using pseudo time-stepping. Hole the aerodynamics of the V-22, engineering development cuts and block-to-block communication is established involving design to improve shipboard suitability, using the x-ray technique established by Meakin13. While survivability, reliability and fleet operations have resulted this technique does not always provide optimal in aerodynamic compromises that caused loss in air- communication among the grid blocks, it does allow the vehicle performance. As a result, design changes were blocks to move relative to each other during a calculation, incurred during Engineering Manufacturing Development for example rotor blades, which has been useful in to regain flight envelope. For instance, external wing applications of OVERFLOW to other V-22 problems. fences, forebody strakes and vortex generators were added to increase low-speed and high-speed performance GRID envelope. Though CFD methods are capable of capturing these aerodynamic interactions, application of these The OML geometry used in this study accurately methods pose a challenge and proper care and modeling is captures all the major components of the V-22. This required; grids designed for specific flow conditions or includes the wings, fuselage, nacelles, and empennage as grid adaptation techniques are sometimes required. well as many of the smaller components of the MV-22B Marine version namely the wing fence, conversion This paper compares independent CFD analyses fairing, forebody strakes, VCD, ARRP, FLIR, AA47 of the MV-22 in airplane mode. Two CFD codes, a fairings, main landing gear door bumps, SATCOM antenna, windshield wiper fairing, landing lights, and structured code, OVERFLOW 2.1, and an unstructured code, FUN3D 10.8 are utilized to generate solutions over glide slope antenna. The surface is shown in Figure 3. a range of angles of attack for conditions corresponding to For FUN3D, the grid bounding this surface consists of mixed elements14, with prismatic layers near the 200-knot flight at 3000 feet. The rotors are not modeled boundaries and tetrahedral elements in the isotropic to simulate the unpowered airframe aerodynamic wind regions. The underlying surface geometry used to tunnel testing. Forces and moments are compared and generate the grid consists of the same PLOT3D surfaces benchmarked against the latest wind-tunnel used to generate the OVERFLOW grid. These surfaces measurements. Interactional aerodynamics and off-body were converted to NURBS surfaces for unstructured grid flow features are also compared. generation. The grid has 15.1M nodes and 55M cells. Table 2. Model-scale flight conditions. The viscous layering was set such that y+ is less than or equal to 1.0. However, when running an 8° prediction, Characteristic Value the maximum y+ was found to be 0.64 at all surface Mach 0.2 locations. An initial tetrahedral grid has been generated Angle of Attack 0°, 4°-20°Δ=2° using GridTool and VGRID. FUN3D has been used to Altitude Sea Level merge the elements in the boundary layer region to create Nacelle Airplane Mode prismatic layers, with pyramids inserted to transition to Flap and Elevator 0° the tetrahedral portion of the domain. Air Properties Standard Reynolds Number 1.5M per ft RESULTS MODEL SCALE The model-scale (MS) CFD study was designed to benchmark FUN3D and OVERFLOW against a particular wind tunnel (WT) configuration which closely Forebody resembles the production MV-22 configuration. The Strake same grid is used for this case as for the full-scale (FS) study. Scaling to the model problem is obtained by VCD adjusting the Reynolds number.
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